Automatic transmissions used in automotive vehicles may include a hydraulic pressure control system for controlling the application of friction clutches and brakes to accomplish gear ratio changes and to effect startup from a standing start in either forward drive or reverse drive. A clutch is used during vehicle startup to establish a torque flow path between an engine-driven turbine of a hydrokinetic torque converter and the torque input gear element of the transmission. The clutch, however, does not include a mechanism for compensating for normal wear of the clutch nor for changes in driveline variables such as temperature changes and engine calibration tolerances. The transmission, furthermore, is not typically calibrated to satisfy the requirements of different drivers with varying driver habits. A smooth clutch engagement for one driver might not occur with a second driver with different driving habits.
Variations, over time, in quality of the clutch engagement may occur also due to normal wear of the valve elements and changes in the characteristics of springs that are used to control valve movements.
Examples of known automatic transmissions that employ startup clutches for multiple-ratio gearing may be seen by referring to U.S. Pat. Nos. 4,527,678, 5,150,297 and 5,157,608, which are assigned to the assignee of this invention. The disclosures of these patents are incorporated in the present disclosure by reference.